The present invention relates to a power supply arrangement, especially in connection with a conductor of a medium-voltage overhead line, which arrangement comprises a power supply and a first and second element made of an electrically conductive material, which are connected electrically to the power supply for feeding energy to the power supply.
For a reliable transfer and distribution of electric energy, it is important that the quality of the transferred energy can be monitored in a reliable manner. For this purpose, the voltages and currents of transmission and distribution lines are measured, whereby possible fault situations, for instance, can be detected and located. It is known to position wireless sensors measuring the current and voltage of conductors on overhead lines, which sensors transmit the measuring information over a radio link to a receiver which interprets the measuring information and possibly forwards it on.
Supplying energy to such wireless measuring sensors has conventionally been done by using batteries as the voltage source. A weakness in such an arrangement is that the batteries must be replaced at regular intervals. It should be understood that changing the batteries of a device connected to a medium-voltage conductor is very arduous and dangerous with respect to safety, since such conductors are typically located at a height of approximately 10 m and their voltage to ground is 6.9 to 21 kV. In addition, the lines are often completely uninsulated or only lightly insulated.
A second known power supply method for wireless measuring sensors in medium-voltage conductors is to utilise the magnetic field generated by the current running in the medium-voltage conductor. Such a method is disclosed in published EP patent 0314850 B1. The publication describes, how an overhead line is used as a primary coil of an iron-core transformer, whereby the required voltage for using the measuring sensor is obtained through a secondary coil of the transformer mounted on the line. A weakness in such a method is that voltage is obtained for the wireless device only when current runs in the conductor. In other words, it is impossible to measure the electric properties of a conductor which has a voltage but no current using the device of the publication. A situation in which a line has a voltage but no current arises for instance when the switching device of the line is opened. In such a case, there is a voltage on the supply side of the switching device, but it cannot be detected by the arrangement described in the publication. Determining the voltage of a currentless line is necessary for instance for monitoring reconnections or the voltage of line sections on both sides of an opened switching device.
JP publication 7250442 A discloses a power supply arrangement connected to a steel tower of a transmission line. The arrangement of the publication is based on a metal plate located in the electric field of the transmission line and having a capacitance to the transmission line. Then when the capacitance charges itself, a potential difference is formed between the metal plate and ground potential. This type of solution thus only functions with towers supporting metal conductors. In addition, in such an arrangement, fixed installations to the tower are necessary, and thus it is not suitable for use as a power supply in devices mounted directly to the conductor.
U.S. Publication 4,931,843 discloses a capacitively connecting voltage sensor which also has a voltage tapping point connected to it for supplying power to a low-power device. In this solution, the voltage tapping point is implemented by a strong capacitive connection from between a metal object connecting to a high-voltage conductor and ground potential. The solution thus requires grounding the sensor galvanically to generate said voltage.
U.S. Publication 5,001,402 discloses an arrangement for burning neon lamps arranged to high-voltage conductors. Generating voltage for said lamps is implemented capacitively in such a manner that one electrode of the lamp is galvanically connected to the high-voltage conductor and the second electrode is connected to a metal surface formed by a capacitive connection. The lighting and burning of the lamps require that the lamps be located in a high electric field and in addition, the lamps should be located so that a parallel capacitance is formed beside them.
It is an object of the present invention to provide a power supply arrangement which avoids the above-mentioned drawbacks and enables the supply of electric energy to the power supply of the arrangement as a unit arranged to a live medium-voltage conductor. This object is achieved by an arrangement of the invention, which is characterized in that said first element is arranged close to the conductor and has a capacitance to an element in a second potential, the capacitance being arranged to charge itself by effect of an electric field generated by the voltage of the medium-voltage conductor, and that said second element is arranged to have a galvanic or capacitive connection with the conductor for feeding energy to the power supply from the potential difference between said first and second elements.
The power supply arrangement of the invention is based on the idea that the energy for feeding the power supply is directly obtained through the ground capacitance of a device suspended from the medium-voltage conductor or possibly through some other capacitance. The current running through the ground capacitance of the device connected to the conductor is then utilised to feed other electronics of the device. The arrangement of the invention provides considerable advantages over prior art. When the arrangement is connected to a medium-voltage conductor, it is a completely maintenance-free functional unit as regards voltage supply, in other words, it does not contain replaceable parts, such as accumulators or batteries. Thus a device containing the arrangement, such as a sensor defining the properties of a conductor, need not be removed from the conductor after it has been mounted there.
In addition, to generate energy, the arrangement of the invention only requires a conductor having a voltage, to which it is connected. That is, the conductor does not necessarily need to have a current. This is why the arrangement of the invention is well-suited for use with wireless measuring sensors, for instance.